· higher data rate than in PSK (2 bits per bit interval), while bandwidth occupancy remains the same

4-PSK can easily be extended to 8-PSK, i.e. n-PSK

• however, higher rate PSK schemes are limited by the ability of equipment to distinguish small differences in phase uses "two-dimensional" signaling

• original information stream is split into two sequences that consist of odd and even symbols

· PSK modulators are often designed using the QAM principle but are not considered as QAM since the amplitude of the modulated carrier signal is constant. QAM is used extensively as a modulation scheme for digital telecommunication systems. Arbitrarily high spectral efficiencies can be achieved with QAM by setting a suitable constellation size, limited only by the noise level and linearity of the communications channel.

Noise immunity of QAM is very high.

· QAM is best suitable for high bit rates.

· Low error probability.

· Baud rate is half the bit rate therefore more effective utilization of the available bandwidth of the transmission channel.